When you look at Rockefeller, Carnegie, Ford, and perhaps even Gates, they had near total control over some aspect of the American Economy. And they used that control to make incredible profits.
Amazon is trying to do the same, but they haven't done so yet. Their retail business is not wildly profitable (if at all). They haven't monopolized online retail (and may never be able to do so).
Amazon may be the next Walmart, but they're not the next Standard Oil. And the market has priced in huge expectations that Amazon will eventually have to meet, or investor exuberance will wane.